Residents said most of the victims were of the majority Hutu tribe. They blamed the predominantly Tutsi army for the massacre.
Aid workers said they saw between 60 and 80 bodies, mostly men but including some women and children, dragged on to roads by outraged residents in the northern district of Kamenge.
'Things are falling apart again. There were many killings in Bujumbura in the last week,' said a Western aid official. 'It is quiet during the day, but after dark the shooting starts. I saw 20 to 30 dead in Kamenge, but there could have been more. Some had been shot while others had been killed with knives. They were predominantly Hutus. The residents said they had put them on the road to protest against this massacre.'
Aid workers said the killings followed an announcement on Saturday by President Cyprien Ntaryamira, a Hutu, that action had to be taken to end violence and the proliferation of weapons.
Up to 100,000 people have been killed in Burundi since its first elected president, Melchior Ndadaye, was murdered by renegade soldiers in a failed coup last October.
A 35-year-old Italian doctor working for the medical agency Medecins Sans Frontieres-Belgium was wounded in the leg by a bullet that hit his car in Kamenge on Saturday. His wife, also a doctor, was hit by metal fragments. It was the first time a foreign aid worker had been shot in the lakeside capital since October.
'In areas where Hutus are strongest they chase out Tutsis with guns and grenades,' said the aid official. 'Vice-versa in areas where Tutsis are the majority. The city is being segregated by force.'
An estimated 650,000 people have fled to neighbouring countries since the violence started in October.